1. Field of the invention
The present invention relates to a cellulose acylate film having small optical anisotropy and capable of being directly stuck to a polarizing film, and a method for producing thereof, and to a retardation film, a polarizer, and a liquid crystal device using the cellulose acylate film.
2. Description of the Related Art
A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation films, polarizers and image display devices. Films that are more excellent in point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely-employed for optical films.
Of those, cellulose ester films having suitable moisture permeability can be directly stuck to most popular polarizing films formed of polyvinyl alcohol (PVA)/iodine in on-line operation. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizers.
When the cellulose acylate film is utilized in optical application such as a retardation film, a protecting film for a supporter of the retardation film and for a polarizing plate, and a liquid crystal display device, the control of the optical anisotropy is a very important point in determination of the display device performance (for example, visibility). Recently, it is required to improve retardation compensation as a wide visual field angle is required in the liquid crystal display device. Accordingly, it is required to appropriately control a value of retardation in an in-plane direction (Re; hereinafter, may be referred to as “Re” briefly) and a value of retardation in a thickness direction (Rth; hereinafter, may be referred to as “Rth” briefly) of the retardation film disposed between a polarization film and a liquid crystal cell. In addition, since the protecting film, which is the film protecting the polarizing plate and is disposed between the polarizing film and the liquid crystal cell, is required to have optical isotropy, that is, to be low in Re and Rth, it has a great effect on the visibility of the liquid crystal display device. Moreover, it is required to appropriately control the moisture absorptivity as well as the optical characteristic on the film.
As a method of producing the cellulose acylate film showing the optical characteristic, there is disclosed, for example, a method using a retardation-lowering agent or a method selecting kinds of polymers (for example, see JP-A-2006-30937). However, in the film produced by these methods, both of the optical property and the moisture absorptivity can not be compatible with each other.
In addition, there is disclosed a method of producing a retardation film by extending a web of cellulose ester dope at a temperature in the range of 140° C. to 200° C. (for example, see JP-A-2003-14933). However, kinds of polymers and a heating temperature are not appropriately controlled in the documents, thus the film in which the retardation is low can not be obtained.